1. Field of the Invention
The present invention relates to a multiaxial optical coupler, and more particularly to a multiaxial optical coupler for achieving optical coupling between end faces of a plurality of optical waveguides and the same number of optical fibers via a lens.
2. Description of the Related Art
The technique of multiaxial optical coupling is essential to an acoustic optical tunable filter (AOTF), a lithium niobate (LiNbO3; LN) modulator, etc. for use in an add drop multiplexer (ADM) which is a data multiplex transmission device arranged at an intermediate portion of a two-way optical transmission path. Multiaxial optical coupling makes it possible to pass optical signals between a plurality of optical waveguides (generally two optical waveguides) and the same number of optical fibers.
Conventionally, there have been proposed e.g. the following multiaxial optical couplers:
FIG. 11 shows a direct fiber coupler applied to an AOTF. A substrate 110 is formed with an optical path of optical waveguides 111a, 111b, 111c and an optical path of optical waveguides 112a, 112b, 112c. These optical paths meet each other at two intersection points. A combshaped electrode 113 is arranged across the waveguides 111b, 112b, for applying RF frequency voltage from an RF frequency oscillator 114 thereto.
Further, an auxiliary member 115 is provided on an upper surface of a left end portion of the substrate 110, for protecting the end faces of the waveguides 111a, 112a. The waveguide 111a is coupled to an optical fiber 101, while the waveguide 112a is coupled to an optical fiber 102.
Similarly, an auxiliary member 116 is provided on an upper surface of a right end portion of the substrate 110, for protecting the end faces of the waveguides 111c, 112c. The waveguide 111c is coupled to an optical fiber 121, while the waveguide 112c is coupled to an optical fiber 122.
To couple the waveguide 111c and the optical fiber 121 to each other, an end of the optical fiber 121 is pressed against the end face of the waveguide 111c directly, and after final adjustment, the waveguide 111c and the optical fiber 121 are bonded to each other. The waveguides 111a, 112a, and 112c are coupled to the optical fibers 101, 102, and 122, in the same manner.
In the AOTF as described above, two optical beams having respective wavelengths .lambda.1 and .lambda.2 are inputted e.g. from the optical fiber 101. When RF frequency voltage having a frequency f1 is generated by the RF frequency oscillator 114 and applied to the combshaped electrode 113, a surface acoustic wave (SAW) is generated over the surface of the substrate 110 to change the direction of polarization of only the laser beam having the wavelength .lambda.1. As a result, the beam having the wavelength .lambda.1 is outputted from the optical fiber 122, and the beam having the wavelength .lambda.2 from the optical fiber 121. Thus, it is possible to take out the beam having the wavelength .lambda.1 alone. Similarly, it is possible to take out the beam of the wavelength .lambda.2 from the optical fiber 122 by applying RF frequency voltage having a frequency f2, which is generated by the RF frequency oscillator 114, to the combshaped electrode 113. The dropping capability of the ADM can be realized by this action.
On the other hand, if an optical beam having a wavelength .lambda.2 is inputted to the optical fiber 101 and an optical beam having a wavelength .lambda.1 to the optical fiber 102, and then RF frequency voltage having a frequency f1, which is generated by the RF frequency oscillator 114, is applied to the combshaped electrode 113, it is possible to obtain the beams of wavelengths .lambda.1 and .lambda.2 from the optical fiber 121. The adding capability of the ADM can be realized by this action.
FIG. 12 shows a conventional V-groove coupler. In this coupler, in the surface of an Si substrate 130 formed with optical waveguides 131, 132, V grooves 133, 134 are formed in a manner extending from ends of the optical waveguides 131, 132, respectively, in the same axial directions. Optical fibers 141, 142 are fitted in the V grooves 133, 134, respectively, whereby direct optical coupling is achieved between the optical fibers 141, 142 and the optical waveguides 131, 132, respectively.
FIG. 13 shows a conventional array lens coupler. In this coupler, an array of microlenses 170 is interposed between a substrate 150 formed with optical waveguides 151, 152 and ferrules 161, 162 containing respective optical fibers 161a, 162a, whereby optical couplings are effected between the optical waveguides 151, 152 and the optical fibers 161a, 162a, respectively, via the microlens array 170. After adjusting the positions of the coupled members for optimization, a metal holder for retaining the microlens array 170 is laser welded so as to prevent displacement of the members from the adjusted positions.
FIG. 14 shows a conventional 2-core ferrule coupler. In this coupler, an aspherical lens 200 is interposed between a substrate 180 formed with optical waveguides 181, 182 and a 2-core ferrule 190 containing optical fibers 191, 192, and optical coupling is effected between the optical waveguides 181, 182 and the optical fibers 191, 192, respectively, via the aspherical lens 200. Similarly to the above array lens coupler, after adjusting the positions of the coupled members for optimization, a metal holder for retaining the aspherical lens 200 is laser welded so as to prevent displacement of the members from the adjusted positions. This coupler may employ a spherical lens instead of the aspherical lens 200.
Generally, in an AOTF or the like, LiNbO3 is used as a material for a substrate on which optical waveguides are formed, and hence there is a need for a multiaxial optical coupling method applicable to substrates of this kind of material. Further, it is desired that this kind of device can deliver a predetermined performance over a wide temperature range.
In the direct fiber coupler shown in FIG. 11, the end faces of the waveguides 111, 112 are bonded to the respective optical fibers 121, 122, by an adhesive. However, the glass transition temperature is in the range of approximately 50 to 60.degree. C. Therefore, if the temperature of the junctions of the waveguides 111, 112 and the optical fibers 121, 122 due to changes in fixing intensity of the adhesive or the like exceed a glass transition point, the end faces of the waveguides 111, 112 can be displaced from the bonded end faces of the optical fibers 121, 122. Now, a device necessitating means for optical coupling which will come into use is expected to have an operating temperatur e range of e.g. 0 to 85.degree. C., so that the conventional direct fiber coupler is likely to cause large insertion loss. For this reason, it is impossible to use the above type of direct fiber coupler.
Further, the direct fiber coupler is unreliable in that an increase in load due to temperature cycling or the like can cause degradation of the bonded portions.
Still further, direct optical coupling produces portions different in refractive index, and hence it is impossible to set return loss to a large value (above 30 dB) after modularization.
In the V-groove coupler shown in FIG. 12, the V grooves 133, 134 are formed on the Si substrate 130. The Si substrate 130 allows the grooves 133, 134 to be formed accurately at a low cost. However, it is impossible to form the V grooves on a LiNbO3 substrate accurately at a low cost.
Accordingly, to apply the coupling method using the V grooves to a coupler using a LiNbO3 substrate, it is required, as shown in FIG. 15, to form V grooves on a Si substrate 220, fit optical fibers 221, 222 in the respective grooves, and then join the Si substrate 220 to a LiNbO3 substrate 210 formed with optical waveguides 211, 212. However, it is extremely difficult to position the Si substrate 220 and the LiNbO3 substrate 210 for proper optical coupling between the optical fibers 221, 222 and the optical waveguides 211, 212, and hence this method is not practical.
In the conventional array lens coupler shown in FIG. 13, it is possible to secure the metal holder retaining the array of microlenses 170 by laser welding, thereby enhancing reliability of optical coupling against changes in temperature. However, the arrayed microlenses 170 which are extremely thin are apt to be warped by thermal load or the like. The warpage of the lenses changes optical paths, and increases insertion loss. Further, stress applied to the lenses by the warpage causes marked degradation of polarization quenching ratio.
The conventional 2-core ferrule coupler shown in FIG. 14, which employs the aspherical lens 200 or a spherical lens, does not suffer from the above-mentioned problems as occurs to the array lens coupler, and hence it is reliable over a wide temperature range. In the 2-core ferrule coupler, however, optical beams transmitted via the optical fibers 191, 192 or the optical waveguides 181, 182, respectively, cannot pass through the center of the aspherical lens 200 or the spherical lens, which results in an increase in coupling loss. This problem comes from the construction of the 2-core ferrule coupler as described below with reference to FIGS. 16 and 17.
FIG. 16 shows the substrate 180, the 2-core ferrule 190, and the aspherical lens 200, as viewed from the top of the coupler. The distance between the central axes of the optical fibers 191, 192 contained in the 2-core ferrule 190 is 125 .mu.m at the minimum. The lens 200 is positioned such that the central axis thereof is opposed to the mid point between the two central axes of the optical fibers 191, 192, so as to equalize coupling losses of the respective optical fibers 191, 192. Therefore, the distance (translational displacement) between the central axis of the aspherical lens 200 and that of each of the optical fibers 191 and 192 is more than 60 .mu.m.
FIG. 17 is a graph showing the relationship between coupling loss and translational displacement of the central axis of the optical fiber from that of the lens. As shown in the graph, when the translational displacement reaches 60 .mu.m, the coupling loss is increased by about 0.6 dB. Therefore, the 2-core ferrule coupler is inconvenience in that it is required to minimize the coupling loss per se, and that it is necessary to adjust the minute distance in adjustment of the optical axes, which makes operations for adjustment very difficult.